912UL Carb sync

Walt wrote:

I made a 1" crossover tube for my 912ULS, It definately helps but you should still balance the carbs. I made adapters to fit into the couplers that the carb balancer fits into.

pics here http://waltsrv12.com/engine/1-inch-balance-tube/

Hi Walt,

The quote below is from the article you supplied

"I believed if the balance tube was big enough there will be shared mixture between both sides of the engine and a smoother run will be the result."

The atmosphere inside the balance tube may contain fuel : air mixture, however this has little to nothing to do with the purpose of the tube.

As many before me have stated - the balance tube is just that, a mechanism for evening out (balance) any vacuum difference between the two intake manifolds. In so doing,  the fuel ; air mixture to each cylinder is equalised, giving smooth (er) power delivery.

Sean, you are certainly entitled to your opinion, I had opinions also until I tried it. I modified a second set so I could switch and compare. The smaller tube can’t transfer air left to right at higher rpm’s, it takes a bigger tube to accomplish that. I don’t think it adds horsepower it just smoothness at the higher rpm’s.

I suppose if you had an engine that had severely different manifold pressures left to right then it would increase the horsepower since left and right wouldn’t fight each other. But that theoretical engine would have issues that should be solved first.

Most auto engines that use separate intake manifolds use much larger balance tubes than the stock Rotax one. 
Ideally if both carburetors fed into one common manifold the engine would run much better. By enlarging the balance tube we are moving in that direction. 

I am way out of my depth here but I put the following logic (at least to me) to you -

Such a simple straight forward, minimal cost, idea - strange that Rotax don't seem to agree with you.

Don't get me wrong I do not think that Rotax have the answer to every question or solution to every problem, that may be associated with their engines however this seems so simple, why has it not been officially adopted?

The balance tube is not about transferring air or fuel, its about equalising vacuum between the two intake manifold pairs. Sure air will move in the direction of the greater vacuum but if the carburettors have been well balanced, this will be minimal. Don't forget that when conducting a pneumatic carburettor balance, the balance tube should be disconnected/blocked off, proving that the tube itself is not required for the engine to run smoothly at all rpm's (subject to successful carb balance).

Sorry  Walt, I just cant see how a larger balance tube will have any significant benefit - but you go for it my friend.

Hello all

Walt you have an intreasting perspective on the compensation tube.  Some of us do not really see how this makes if fly better without any physical testing and data.  

My view is what was found in early development and also what I know from my hot rod days as a kid.  Engines with long intake runners tend to have better torque numbers but will run out of air fast at high RPM from increased resistance.  When you want high RPM we used must shorter runners (intake manifolds) to get a better feed into the head.  Rotax could not make peak power unless they used dual carburetors on the original 912.  A single carb simply would not work with the long intake required.  The pain of getting peal power is we needed to use dual intakes with short runners.  To fix the early 912 iS powerband that was exactly what was changed to get more midrange torque, a complete new intake and airbox. (gain of 5 HP in mid range 912iS to 912iS Sport)

What you are seeing in your large tube crossover is something different, it is not actually manifold balancing to the extent you believe.  What is happening is called "Helmholtz resonance" , yes you should perhaps look it up.  This is used in many engines and it acts like a ressovor for air that the engine can use when you try to accelerate and it can't get enough air.  The principles are relevant to intake and exhaust systems.  

I hope someone with more expertise on the matter chimes in, my knowledge is limited to old hot rods and Rotax. 

Cheers

So yesterday I whipped up some adaptors to go from 1" to 1/4" hose barb & connected my gauges. 

It was quite difficult to get to the carbs with the prop on & engine running. Luckily with the design of this plane I was able to get in the backseat on my knees & pop my head & shoulders through the roof via the removable lexan & the carbs were right in front of me. 

With the plane properly secured & my son at the controls I was able to get a really good sync on the carbs. They were off quite a bit. It seems to run really well right now. I need to get in a flight before I can confirm 100%, but the ground run was promising. 

Thanks for all the help!